It happens to the best of us — even Paula Radcliffe. In 2005, on her way to winning the London Marathon, the women’s world record holder was forced to stop on the side of the road and take a dump in front of thousands of fans. She went on to win the race after pooping, saying that cramps had been bothering her.

Unfortunately, Radcliffe’s experience isn’t uncommon for runners — though most of us make it to a toilet. A review in The International SportMed Journal about gastrointestinal (GI) problems in runners reported that studies have found between 30-83 percent of runners are affected by GI disturbances. One separate study of long-distance triathletes competing in extreme conditions even found that 93 percent of them had at least one symptom of GI distress.

“GI issues are certainly a race day and long run day problem,” said Darrin Bright, the medical director for the Columbus Marathon, who also runs and coaches. The most debilitating and annoying of these GI issues? The sudden and overwhelming need to evacuate your bowels. In cases of extreme frequency or discomfort, this is known as runner’s diarrhea.

While the exact cause of runner’s diarrhea isn’t pinned down, there are a few things sending people running to the bathroom.

“Contributing factors likely include the physical jostling of the organs, decreased blood flow to the intestines, changes in intestinal hormone secretion, and pre-race anxiety and stress,” said Dr. Stephen De Boer, a registered dietician with the Mayo Clinic who has studied this topic. “What is clear is that food moves more quickly through the bowels of athletes in training.”

As you run, all that stuff inside your GI tract gets shaken and loosened. The lack of blood flow in the intestines and changes inside your body force all that stuff out. And getting stressed about it won’t help.

“A lot of people carry stress in their GI tract,” said Bright.

The good news is that for most people these issues are relatively minor. Having to make a stop at the port-a-potty before a race is completely normal. It only becomes a problem when your toilet stops are getting in the way of your run.

To avoid the need to pull a Paula, Bright advises trying to get your system going beforehand. That can be as simple as having a little food or coffee to activate the bowels and heading to the bathroom before you hit the road. Even eating or drinking something small can activate the gastrocolic reflex, which essentially pushes things out when new food comes in.

Or, a short run warm-up in advance of a race can be enough to get the bowels moving and help you empty everything out before the start, which is by far the best way to avoid having to stop later. There’s a reason the port-a-potty lines before races are always so long. If you’re heading for a run, not a race, the best idea might be to plan a route with a bathroom stop near the beginning or doing a quick out-and-back to hit your home toilet after warming up.

The other common precaution many runners take is to avoid foods that pass through the system quickly, such as apple juice, and foods with high fiber that can cause the need to defecate.

“At least one day before running, limit or avoid high-fiber and gas-producing foods, such as beans, bran, fruit and salad. If you run every day, experiment to find a tolerable level of fiber. Otherwise, simply eat those foods after you run,” wrote De Boer, with Dr. Edward Laskowski, previously on the subject.

If you are particularly susceptible to problems, then De Boer and Laskowski also suggest avoiding high-fat foods, caffeine, and sugar alcohols. Most people also avoid all food within the two hours immediately before an event. During an effort, it’s important to stay hydrated. However, warm liquids have been known to “speed foods through the digestive tract,” wrote De Boer. For some people, energy gels and bars can cause problems.

All that means that finding what works for you is somewhat “trial and error,” said Bright, who noted it’s taken him some time to find his own pre-race and race nutrition routine. Practicing your nutrition on long runs has the added benefit of helping to relieve race stress, which contributes to GI problems.

If, for some reason, this continues to be a larger problem, then Bright suggests going to see a doctor. “It could be a sign of something more serious going on,” he said.

You can also take comfort: all that rushing to the bathroom could be reducing your risk of colon cancer. The rate of colon cancer has been found to be lower among runners, which may be because their bowel content is not present in their colons as long as it is with non-runners.